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J/A+A/526/A102 VLBI imaging of 105 extragalactic radio sources (Bourda+, 2011)
VLBI observations of optically-bright extragalactic radio sources for the
alignment of the radio frame with the future Gaia frame.
II. Imaging candidate sources.
Bourda G., Collioud A., Charlot P., Porcas R.W., Garrington S.T.
<Astron. Astrophys. 526, A102 (2011)>
=2011A&A...526A.102BADC_Keywords: VLBI ; Interferometry ; Radio sources ; QSOs ;
Active gal. nuclei ; BL Lac objects
Keywords: reference systems - quasars: general - astrometry - catalogs -
methods: observational - techniques: interferometric
Abstract:
The European space astrometry mission Gaia, to be launched by 2012,
will construct a dense optical QSO-based celestial reference frame
which will need to be linked to the International Celestial Reference
Frame (ICRF; the IAU fundamental frame), with the highest accuracy.
However, it has been found that only 10% of the ICRF sources (70
sources) are suitable to establish this link. The remaining sources
are not useful either because they are not bright enough at optical
wavelengths or because they have significant extended radio emission
which precludes reaching the highest astrometric accuracy. In order to
improve the accuracy of this alignment, we have developed a program of
VLBI observations based on three steps to detect, image and measure
astrometric positions of weak extragalactic radio sources, with bright
optical counterparts, from a sample of 447 candidate sources. The
experiments devoted to VLBI detection, carried out with the European
VLBI Network (EVN) in June and October 2007, were very successful,
with 398 sources detected at both S- and X-bands. From these, 105
sources were observed in March 2008 with a global VLBI array (EVN and
VLBA; Very Long Baseline Array) for imaging their VLBI structures. All
sources were successfully imaged in both bands and about 50% (47
sources) were found to be point-like on VLBI scales. These images are
available at http://www.obs.u-bordeaux1.fr/BVID/GC030/. VLBI positions
of these sources will be measured accurately in future astrometric
experiments.
Description:
The global experiment GC030 was carried out in March 2008, during
48-hrs, with the European VLBI Network (EVN) and the Very Long
Baseline Array (VLBA) recording at 512 Mbps in a geodetic-style
dual-frequency S/X mode (2.3 and 8.4GHz). These VLBI observations
were dedicated to imaging 105 weak extragalactic radio sources, with
bright optical counterparts, detected during previous VLBI experiments
(Bourda et al., 2010A&A...520A.113B).
The list of 105 target sources is detailed in this table, as well as
the total flux densities as integrated from the X- and S-band images
(units in mJy), the corresponding S/X-spectral index, the continuous
structure index at X- and S-bands, and the parameters of the naturally
weighted VLBI images at both bands. These latter are:
(i) the restoring beam, which is an elliptical Gaussian with FWHM
(Full Width at Half Maximum) major axis a and minor axis b (units in
mas), with major axis in position angle phi (measured north through
east; units in degrees);
(ii) the peak brightness and the rms (root mean square) of the
residuals of the final hybrid image (units in mJy/beam);
(iii) the first contour level of the image (i.e. percentage of the
peak and corresponding value in mJy/beam).
File Summary:
FileName Lrecl Records Explanations
ReadMe 80 . This file
table3.dat 72 210 List of the 105 optically-bright, weak extragalactic
radio sources, observed during GC030
at X- and S-bands
See also:
J/A+A/520/A113 : 398 extragalactic radio sources VLBI detection (Bourda+, 2010)
http://www.obs.u-bordeaux1.fr/BVID/GC030/ : Bordeaux VLBI Image Database
Byte-by-byte Description of file: table3.dat
Bytes Format Units Label Explanations
1- 8 A8 --- IERS IERS designation (HHMM+DDd)
11 A1 --- Band [XS] Band of observation: X=8.4GHz, S=2.3GHz
14- 18 F5.1 mJy Flux Total flux density integrated from the image
21- 24 F4.1 --- alpha S/X spectral index αSX
27- 29 F3.1 --- SI Continuous structure index (1)
32- 35 F4.2 mas a Beam major axis (2)
38- 41 F4.2 mas b Beam minor axis (2)
44- 48 F5.1 deg phi Beam major axis position angle (2)
51- 55 F5.1 mJy Peak Peak brightness (in mJy/beam)
58- 61 F4.2 mJy RMS RMS of the residuals of the final hybrid
image (in mJy/beam)
64- 66 F3.1 % FCL% First contour level: percentage of the peak
69- 72 F4.2 mJy FCL First contour level: value (in mJy/beam)
Note (1): the structure index is defined as SI = 1 + log(τm),
where τm represents the median value of the structure delay
corrections in picoseconds (ps=10-12s). Only sources with values
SI<3.0 should be used for the alignment of the VLBI and Gaia frames.
Note (2): The restoring beam is an elliptical Gaussian with FWHM (Full
Width at Half Maximum) major axis a and minor axis b (units in mas),
with major axis in position angle phi (measured North through East;
units in degrees).
Acknowledgements:
Geraldine Bourda, Geraldine.Bourda(at)obs.u-bordeaux1.fr
References:
Bourda et al., Paper I 2010A&A...520A.113B, Cat. J/A+A/520/A113(End) Geraldine Bourda [LAB, France], Patricia Vannier [CDS] 23-Nov-2010